Explosives are substances capable of exerting, by their characteristic high-velocity reactions, sudden high pressures. Chemical explosives are divided into two main categories, the "low-order" or "deflagrating" type and the "high-order" or "detonating" type. The latter are further classified as "primary" or "secondary" detonating explosives.
Deflagrating explosives are characterized by a reaction rate which increases nearly in direct proportion to the pressure (as a result of the influence of pressure on surface temperature), but always remains one or two orders of magnitude lower than the reaction rate in the detonating type. The explosion typically moves through the unexploded material at a speed slower than that of sound in that material. The limiting rate of reaction and pressure in granular low explosive is determined by the effective burning surface and the upper limit of surface temperature. The pressure-time curve of a deflagrating explosive tends to exhibit a maximum usually below about 75,000 psi and normally around 50,000 psi.
In contrast, detonating explosives are characterized by an explosive process in which the reaction takes place within a high-velocity shock wave known as the "detonation wave" or "reaction shock." This wave generally propagates at a constant velocity, typically faster than the speed of sound in that material, depending on the chemistry of the explosive, its density and its physical state. Pressures generated by detonation range from about 1.5 million to 4.5 million psi.
Primary detonating high explosives are used to detonate other high explosives. The reaction in a primary explosive is initiated by heat or shock waves, and such explosives are extremely dangerous because of their high sensitivity. They first burn or deflagrate for a few micro-seconds, then detonate.
Secondary detonating high explosives produce the largest amounts of energy. Without initiation by a primary high explosive, they are relatively stable. Detonation of the explosive depends on its confinement, the rate of heat dissipation, and the nature of the explosive itself.
There are a variety of chemical explosive compounds, each one with characteristics that determine the conditions under which it can advantageously be used. Accordingly, a particular explosive compound may be more desirable for use in one situation than in another, a different explosive compound being better suited for use under the latter situation's conditions. However, all types of explosives have at least one characteristic in common: they require some sort of activation, by application of externally supplied means such as heat, flame, electrical discharge, impact or shock, to initiate the explosive reaction. It nonetheless confirms their diversity that sensitivity to the aforementioned stimulus varies from one explosive to another, and varies even for a given explosive under different conditions of temperature, pressure, concentration, density and physical state.
Explosive charges, both of the deflagrating and detonating type, are utilized for various functions in the oil and gas industry; one frequent use is for perforating a well casing to complete or test a formation, and another is for setting a packer or other device downhole in a wellbore. Due to the time and the expense involved in carrying out such operations and the explosive power of the compounds, it is essential that the performance of the explosives be reliable. Furthermore, it is important that explosive materials be resistant to the extremes of temperature encountered in the typical wellbore environment because such conditions can degrade the operation of the explosive materials.
Because of the difficulty in setting up and maintaining electrical means connected to an explosive charge to cause electrical initiation of an explosion within the wellbore--due largely to its depth--it is desirable to ignite low-order explosives and detonate high-order explosives by an initial deflagrating explosive charge which is actuated by impact. However, that percussive actuation also poses problems. Deflagrating explosives which are known to ignite upon impact and therefore might be utilized as initiators, such as those generally used to fire bullets and other projectiles, including lead thiocyanates and barium styphnates, contain organic moieties which introduce instability under extremes of temperature over periods of time. Furthermore, such compounds are extremely sensitive, igniting upon an impact of only 1 to 20 inch-ounces. That sensitivity could well cause premature firing of the explosive under the harsh conditions within the wellbore, with the result that the wellbore would be damaged so as to require difficult repairs or even permanent closure.
Other percussion-activated explosives used in the oil and gas industry, such as lead azide and lead styphnate, also are extremely sensitive to impact ignition and show poor stability at various extremes of temperature.
As a result of the shortcomings of known percussive primer mixtures, the art would ordinarily be inclined to utilize instead the aforementioned electrically initiated primers. One such primer contains a mixture of titanium and potassium perchlorate. Thus, in a percussive primer the mixture of titanium and potassium perchlorate would typically be disposed in cooperation with an electrical heating element, which element imparts heat energy to the mixture causing it to ignite (for instance, at around 750.degree. F.). Compaction of the titanium and potassium perchlorate mixture is unnecessary to and does not in any appreciable manner improve the essential capacity of the mixture to function as a primer. While in practice the mix is compacted at a pressure of around 15,000 psi to a density of around 2.2 g/cc, this is done only so that 1 watt of heat, a value often incidentally encountered in operation, can safely flow through the heating element and be dissipated without igniting the primer mix. Increasing the heating power input to around 5 watts will provide sufficient heat to ignite the primer mix.
However, as previously explained, the disadvantage of electrical ignition discourages the use of such mixture in their known physical state in applications within wellbores.
Provision of a percussion primer and primer mix which confer on the art the advantages accruing with use of a percussion primer, but eliminate the previously discussed problems associated with known percussion primer mixes, would be a highly desirable advance over the current state of technology.